A听& R The Artists and Repertoire (or Artists and Recording) department of a record company. The A&R manager selects acts and oversees the recording process. If you鈥檙e an singer-songwriter you鈥檙e going to pray one of these fellas turns up at one of your gigs.
A Cappella "In the manner of the chapel". Sung music without instrumental accompaniment. Think The Flying Pickets and you get the idea.
Acoustic An instrument that provides sound without the using electronic amplification. Typically, an acoustic guitar and a drum kit in Pop Music.
Acoustic Guitar A guitar which isn鈥檛 plugged into an amp, basically. The body of the acoustic guitar acts as a sounding board and an amplification chamber - as do the bodies of all acoustic stringed instruments. The sound is produced by the strings and the "wooden" and hollow nature of the instrument's body - a good example is "Wonderwall" by Oasis. Playing "Unplugged" is seen as going back to basics and has seen a bit of resurgence in recent years.
Amplifier Electronic devices that are used to increase the volume of sound. In live performance amplifiers are used for guitars, vocals, keyboards, and any instrument that can be miked (linked to a microphone). The stuff that bands use is, in principle, identical to your average home Hi Fi, just a lot more powerful (and expensive).